Since a magnesium diboride (MgBr2) film has relatively high superconducting properties, such as a maximum transition temperature of 39K, among various intermetallic compounds, it is anticipated to be used as a functional material in various fields such as a superconducting magnet, a Josephson element, and a sensor, and fundamental research on physical properties of the magnesium diboride film has also been carried out. MgB2 is superior to an oxide superconducting material in terms of stability and safety and is an intermetallic compound which can be easily handled.
In order to form a magnesium diboride film, as a method using high volatility of Mg, a method has been known in which a precursor film composed of boron or boron containing Mg is crystallized by firing at a high temperature in an Mg vapor atmosphere (JP2004-176174A, U.S. Pat. No. 6,773,836B). However, by the high temperature firing, the structural change caused by oxidation and/or re-evaporation of Mg cannot be avoided, and as a result, the formation of a superconducting device including a multilayer structure and/or a Josephson junction is adversely influenced.
As a method for eliminating defects caused by the structural change, there may be mentioned an as-grown film formation in which MgB2 is grown by vapor co-deposition using an MBE method (JP2003-158308A, JP2004-99347A). However, in order to suppress the oxidation and/or the re-evaporation of Mg, an Mg flow rate must be increased. Furthermore, in order to increase the superconducting transition temperature and the crystallinity, although the temperature is low as compared to that of the firing, the substrate must be maintained at a high temperature during the formation as is the case in the past. In addition, in a method in which H2, N2, and the like are supplied in order to facilitate reaction between Mg and B (JP2004-99347A), it is difficult to obtain a high vacuum condition which is necessary to stabilize the crystallinity of a vapor deposition film.
In a conventional as-grown film formation, the degree of vacuum of film forming conditions is liable to decrease concomitant with an increase in flow rate of an Mg vapor. When the degree of vacuum decreases, adverse influences due to residual gases occur, and as a result, the crystallinity and the orientation of a magnesium diboride film may be degraded in some cases. In addition, in the vapor deposition performed on a substrate which is maintained at a high temperature, a diffusion reaction is facilitated at the interface between the substrate and the thin film, and the interface of the multilayer structural film, which is particularly necessary for device formation, cannot be made steep flat interface; hence, it is difficult to obtain a thin film which can satisfy the properties necessary for device formation. Furthermore, since there have not been sufficient knowledges of modification of a substrate surface for film formation and of conditions forming a B-rich film to improve the properties, it has been difficult to obtain a high-performance magnesium boride superconducting thin film having superior crystallinity.
In recent years, the use of an HPCVD method has also been proposed (U.S. Pat. No. 6,797,341B). Although the HPCVD method advantageously forms a thin film having high crystallinity, because of the limitation from a process point of view, the above method is not suitably used to form a multilayer structural film necessary for device formation.